So, for those of you (like me) who didn't quite understand why sitting in the middle of the horses back is normal for the sport of Saddleseat, I found an excellent article that made it clearer for me. I always thought it was one of those "corrupt horse world" things (you know, soring, heavy shoes, controversial training methods, etc).
Taken from ponies-in-the-house on Tumblr (I wen't through and fixed some grammar mistakes, but the message is the same).
How Horses are Built:
Many people ask me why only certain breeds have Saddleseat riders. Well look at the photos!
“Saddleseat” type horses are built VERY differently than the “typical” horse. Their necks are much higher on their body, this is why they always have that HUGE crest in the hunter and western rings.
They are also built for they legs to move in a more “up and down” motion than a “forward” motion.
Their center of gravity is in fact, in the center of their backs, not on the withers, thus making bareback MUCH easier (don’t believe me? Get on a TB bareback, and then jump on a Morgan. I’m sure you will notice a difference). These horses are built for MOTION not SPEED
The necks much higher on the body are why Saddleseat horses tend to have back problems, being particularly inclined to swayback. The same problems are evident in a lot of draft horses bred for driving as well, since draft horses also tend to be selected for a higher-set neck. In addition, just looking at the illustration, you can see how Thoroughbreds are selected for a flatter topline whereas Saddlebreds are selected for a concave topline. That contributes to back issues too.